Apple’s and Android’s stronghold on the U.S. smartphone market has been the case for many quarters already, and figures out today from Nielsen show how that has translated into a domination of the business of mobile content as well: phones based on Google (NSDQ: GOOG) Android and Apple (NSDQ: AAPL) iOS together account for 71 percent of all smartphones in the U.S., and some 83 percent of all apps that have been downloaded in Q3, according to new figures from the research group.

The two platforms have been gaining users in a crucial period of growth for smartphones as it picks up early adopters: penetration in the U.S. now stands at 44 percent, more than double the 18 percent penetration in Q3 2010. That still means that there is still more than half the market to play for — an opportunity companies like Nokia (NYSE: NOK) and Microsoft (NSDQ: MSFT) hope to catch.

Nielsen, which today publishes its latest State of the Media report, notes apps have remained a popular source of mobile content, with the number of smartphone users downloading apps now at 62 percent. That population is largely concentrated on the two leading platforms, which together account for 83 percent of all app downloads.

The growth on iOS and Android, combined with the declines on other platforms, point to how app ecosystems appear to be coalescing around the dominant players:

— On Android, 49 percent of smartphone owners have downloaded apps, a massive rise on the four percent that had done so in 2009.

— Apple has the second-strongest platform friendly for apps, with 34 percent. But that is actually a slight decline on 2009, when 37 percent of iOS owners downloaded an app.

— But when it comes to other platforms, the percentages of users downloading (and presumably using) apps has actually gone down. Eleven percent of BlackBerry users have downloaded apps, down from 32 percent in 2009. And only three percent of Windows Mobile users have downloaded apps, compared to 18 percent in 2009. The WebOS platform has seen a decline to one percent from five percent, while Symbian remained level at one percent in both years.

What would have been interesting to see is how Windows Phone users rank in terms of app downloads today. Microsoft has made a huge effort to court developers to make apps for the platform and it has seen rapid growth in terms of the number of apps in its Marketplace catalog. But the platform and its app store were not around in 2009, so that may be why Nielsen chose not to include it.

Taken together there are around one million apps on the market today, but the vast majority of those are on Apple’s iOS and Android, which both claim around half a million apps each. Microsoft’s Marketplace annouced 40,000 apps in November 2011.

Overall, content is still weighted to mobile internet, not apps. Although apps downloads are growing, they remain less popular than plain old mobile internet usage, according to Nielsen’s figures. The amount of consumers accessing mobile internet sites in Q3 was 57 million, while app downloads (and again, presumably usage as well) were at 49 million across all smartphone platforms. On individual platforms, like Android, apps are actually more popular than mobile web:

Nielsen notes that games have been the most-downloaded type of app in the last 30 days, and indeed, game downloads overall have grown the most of all “rich media” uses on mobile devices, rising 83 percent versus Q3 2010. That still only works out to 35 million of all smartphone subscribers downloading games, which points to how those who are into games tend to be engaged and probably repeat users.

The other gaming category charted by Nielsen, online game playing, grew the most of any category, at 95 percent, but it is also the smallest, with 16 million of all users engaging in online gaming.

Mobile video, still seen as a great opportunity by content players, is still not seeing huge use compared to other types of content: 31 million people used mobile video in Q3 2011.

Mobile music services have been a particular focus in the last few months, with the launch of Spotify in the U.S., Apple rolling out new features for iTunes and totally new companies like Rara.com wading into the waters — among many other developments. Although it’s still early days for streaming services — there are only about five million people paying for digital music of any form today on mobiles, according to figures from Omnifone and Loudeye — music streaming is more popular than downloads, at 29 million versus 18 million of all consumers using the services; and both are growing very fast, at 66 and 65 percent, respectively. If you combine the two kinds of music offerings, it works out to 47 million of all users engaging in mobile music services, nearly as many as downloading apps of any kind.

International. Nielsen’s monthly reports usually focus on what is happening in the U.S. market — and that’s the case here, too, but this time around Nielsen has also included some comparative figures from other markets. Among them:

— In one graphic charting notable mobile usage in different markets, Nielsen points out that the UK has smartphone penetration of 38 percent, a fair bit lower than 46 percent figure announced by UK regulator Ofcom yesterday but still among the leaders world-wide.

— Argentina has the highest smartphone and multimedia phone penetration in Latin America, at 60 percent.

Nielsen’s survey is based on device metering data from “thousands” of consumers; detailed, monthly analysis of bills for 65,000 mobile customers in the U.S.; and surveys covering around 300,000 consumers each year.

Perhaps given the recent wave of interest in privacy issues around companies like Carrier IQ (who once ran a trial with Nielsen) and how data is mined by companies for research and other purposes, Nielsen makes the point of noting that all its data is volunteer-, opt-in based.